Our development of a 20-mm probe has increased the sensitivity of Fourier transform NMR to the point that it is now practical to observe single-carbon resonances in natural-abundance carbon-13 NMR spectra of proteins. In general, folding of a protein into its native conformation produces sufficient chemical shift nonequivalence for resolving numerous resonances of individual nonprotonated aromatic carbons. We propose to assign to specific residues the resolved nonprotonated aromatic carbon resonances of various lysozymes, cytochromes c, myoglobins, and hemoglobins. We describe the methods that will be used for such specific assignments. We propose to use the specifically assigned resonances to monitor individual atomic sites of native proteins in solution. We propose to investigate the individual pKa values of tyrosine residues of myoglobins and hemoglobins. We propose to study the titration behavior of the numerous histidine residues of various myoglobins. We propose to use the resolved C gamma resonances of individual tryptophan residues of hemoglobins in various states for monitoring the Trp environments. We propose to study the sensitivity of the conformation of native proteins to various perturbations (specific chemical modifications, for example). We propose to evaluate the applicability of natural-abundance C13 NMR in very early stages of structural studies of a protein of unknown (or partly known) composition and sequence. Papaya lysozyme will be used as a test case for this purpose.